1. Field of the Invention
This invention relates to a copolymerization process of propylene, and more specifically to a process for the reutilization of recovered unreacted propylene and ethylene.
2. Description of the Prior Art
It has been well known to improve the impact resistance of polypropylene, especially, its impact resistance at low temperatures by copolymerizing propylene with ethylene. It has also been known to separate a mixture of unreacted propylene and ethylene into an ethylene-propylene mixture and propylene for their recovery and reutilization (see, Japanese Patent Publication No. 42379/1972). This recovery and reutilization of such unreacted propylene and ethylene are extremely beneficial when producing a block copolymer of propylene by the reaction of ethylene and propylene at a relatively high ethylene/propylene ratio, particularly, when producing the block copolymer by bulk polymerization while using propylene itself as a reaction medium.
The above process is however accompanied by a drawback that in order to use a recovered ethylene-propylene mixture, the recovered ethylene-propylene mixture has to be mixed in advance with a fresh supply of propylene or ethylene to adjust the concentration of ethylene to a predetermined constant level or the amounts of ethylene, propylene and/or an ethylene-propylene mixture to be charged have to be controlled in a cumbersome manner to maintain the concentration of ethylene in a polymerization tank at a prescribed constant level because the concentration of ethylene in the recovered ethylene-propylene mixture varies depending on the concentration of ethylene in the corresponding mixture of unreacted propylene and ethylene when a distillation column is operated under constant conditions.